The paper evaluates the contribution of morphological factors to the probability of sentence boundaries in Russian unscripted monologue. The analysis is based on multiple expert manual annotations of unscripted speech which allow obtaining fine-grained estimates of the probability of sentence boundary at each word junction. We used linear regression analysis to explore whether there is a relationship between sentence boundaries marked by the annotators and the grammatical features of the text. We focused on morphological factors related to the presence or absence of sentence boundaries.